1. Field of the Invention
The present invention relates to a torque detecting apparatus used to detect a rotational torque applied to a steering member for steering in, for example, an electric power steering apparatus.
2. Description of Related Art
In an electric power steering apparatus which assists steering by driving a steering assist motor according to the rotational operation of a steering member for steering (for example, a steering wheel) and transmitting the rotational force of the motor to a steering mechanism, it is necessary to detect a rotational torque applied to the steering member for use in drive and control of the steering assist motor. In order to detect the torque, a torque detecting apparatus constructed at the middle of a rotary shaft (column shaft) connecting the steering member and the steering mechanism has been conventionally used.
In the torque detecting apparatus, the column shaft to be detected is divided into a first shaft and a second shaft which are connected on the same axis through a torsion bar with a small diameter. When the steering member for steering is rotationally operated, relative angular displacement occurs between the first and second shafts with a twist of the torsion bar caused by the function of the rotational torque. The torque detecting apparatus is constructed to detect the rotational torque by this relative angular displacement.
The detection of relative angular displacement is realized by, for example, a structure in which detection rings are externally fitted and fixed near the connected section of the first and second shafts, a sensor coil (sensor unit) is fitted and retained in a cylindrical housing supporting the column shaft so as to surround the facing section of the detection rings, a magnetic circuit passing through the facing section of the detection rings is formed by distribution of electric current to the sensor coil, and a change in the facing relationship between the respective detection rings corresponding to the relative angular displacement between the first and second shafts caused by the function of the rotational torque is fetched as an impedance change in the sensor coil due to an increase/decrease in the magnetic resistance of the magnetic circuit, through a circuit board mounted in a board chamber provided inside the housing (see, for example, Japanese Patent Application Publication No. 7-21433 (1995)).
The board chamber in which the circuit board is mounted is connected through a lead aperture to a sensor retaining hole formed to retain the sensor coil in the housing, and the sensor coil and the circuit board are connected through a connection lead which is extended from the sensor coil, passed through the lead aperture and guided into the board chamber.
The mounting of the torque detecting apparatus constructed as described above is realized by a procedure in which the sensor coil as the sensor unit is fitted into the sensor retaining hole, and fixed and retained in a predetermined position, the circuit board is mounted in the board chamber outside the housing, and the circuit board and the sensor coil are connected through the connection lead passed through the lead aperture as mentioned above.
In such a mounting procedure, however, the fitting of the sensor coil into the sensor retaining hole through an opening on one side in the axial direction and the mounting of the circuit board in the board chamber from outside in a radial direction of the housing are the operations of separate processes. Moreover, during these operations, it is necessary to connect the connection lead passed through the lead aperture to the circuit board. Thus, such a mounting procedure has the problem of requiring a number of mounting processes.
Further, since the torque detecting apparatus mounted as described above is used under severe condition in which external force such as vibration caused by driving of a vehicle is always applied, there is a possibility that connection defects may occur with the passage of time in the connection part of the circuit board and the sensor coil due to the function of the external force. In this case, there arise troubles in fetching the output of the sensor coil, and it becomes impossible to perform normal torque detection.
The torque detecting apparatus used for detecting a rotational torque applied to the steering member of an electric power steering apparatus is not limited to the above-mentioned structure comprising the detection rings externally fitted and fixed on the column shaft and the sensor coil fitted and retained in the sensor retaining hole of the housing, and has been applied to practical applications in various structures. However, in any of the structures, the sensor unit retained in the sensor retaining hole of the housing is connected to the circuit board mounted in the board chamber isolated from the sensor retaining hole to fetch the output of the sensor unit through the circuit board, and therefore it is very important to facilitate the mounting of the sensor unit and the circuit board in the housing, to increase the reliability of the connection part of the sensor unit and the circuit board, and to prevent occurrence of connection defects due to the function of external force such as vibration.